Going out and trying to get work is massively annoying. So is context-switching. Plus paying your own benefits, dealing with non-paying customers, legal fees (yes, you need a lawyer if you're going to freelance) and having a complete lack of job security. I don't envy freelancers when I look at what they have to put up with. With all this, a typical hourly rate (and this is generally considered fair) is 1/800 of salary.
He also has an existing project that he's working on full time, and that he'd presumably be taking time away from to work on the consulting gig. When you're already working full-time, it makes sense to tack a factor of 3 on to your hourly rate, on the supposition that leisure time is worth (when you work full time and have little of it) 3 times as much as regular work time.
Given all this, a reasonable consulting rate might be 1/250 of annual salary, so he's rating himself as being at the same level ($250k/year) as a mid-career Wall Street quant. I don't think that's unreasonable.